1. Field of the Invention
This invention relates generally to a circuit model for estimating state-of-charge (SOC) for a battery on a vehicle and, more particularly, to a circuit model that estimates state-of-charge for a battery on a vehicle while the vehicle is operating, where resistances within the circuit model are variable based on voltage.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combines a battery and a main power source, such as an internal combustion engine, fuel cell system, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead acid, etc. The battery can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
Batteries play an important role in powering electrical vehicles and hybrid vehicles. Lithium-ion batteries have proven to be promising for hybrid electric vehicles. The effectiveness of battery control and power management is essential to vehicle performance, fuel economy, battery life and passenger comfort. For battery control and power management, two states of the battery, namely, state-of-charge (SOC) and battery power capability, need to be predicted, or estimated, and monitored in real time because they are seldom measurable during vehicle operation. Battery state-of-charge and battery power can, however, be estimated from the measured battery current and voltage by solving for the parameters in an equation that describes an equivalent circuit model of the battery. These parameters include an ohmic resistance associated with the electron conductors within the cell, RC pairs to describe the electrochemical reaction at the electrode-electrolyte interface and the effects of diffusion in the solid and liquid states, and an open circuit voltage (OCV) that varies with the concentrations of reactants and products involved in the reaction. The OCV may be directly converted to an estimate of state-of-charge, and the circuit model equation may be arranged and solved for power capability when the values of the other parameters are known. Many battery state estimation algorithms based on the premise of circuit model representations have been developed in the art using different methodologies and some have been implemented in vehicles.
It is well known that battery dynamics are generally non-linear and highly dependent on battery operating conditions. However, for on-board battery parameter estimation, a linear model that has a few frequency modes is used to approximate a battery's dominant dynamics for a specific application, such as power prediction or SOC estimation. The reason for this is mainly due to limited computational power and memory available for on-board applications. In fact, even if there was unlimited computational power and memory, an accurate estimation of all battery parameters in a complex model with as many frequency modes as possible cannot be guaranteed because some information about the full range of dynamic behavior may be lacking from the measured terminal voltage and terminal current for any given level of observed excitation. Therefore, it is neither practical nor necessary to cover all frequency modes in one model as long as the estimation error caused by model uncertainties is within an acceptable range for a specific application.
U.S. patent application Ser. No. 11/867,497, filed Oct. 4, 2007, now published as Patent Application Publication No. U.S. 2009/0091299, titled Dynamically Adaptive Method For Determining The State of Charge of a Battery, assigned to the assignee of this invention and herein incorporated by reference, discloses a method for determining battery state-of-charge and battery power using four battery parameters, namely, the battery OCV, ohmic resistance, and the resistance and capacitance of an RC pair.
It is common practice to measure the voltage of a battery that has been at rest for a significant period of time and to convert this voltage measurement to an estimate of state-of-charge. In this situation, the measured terminal voltage is regarded as being equal to the open circuit voltage (OCV), and is directly observable, so the solution of a complex circuit model equation is unnecessary. However, if there are loads on the battery, which is normally the case when the vehicle is operating, it is necessary to remove or subtract from the measured terminal voltage the loss of voltage caused by those loads in order to estimate the OVC. For a battery SOC estimation model using an equivalent circuit, the model is complex and must be accurate so that the voltage losses attributed to various elements of the circuit model can be accurately removed from the terminal voltage to get a proper estimation of the battery SOC. The above mentioned simplified circuit model of the battery polarization is generally effective for calculating battery SOC. However, during certain operating conditions, such as low temperature, high or low battery SOC, etc., the complexity of the actual battery chemistry does not allow the simplified circuit model to accurately represent those conditions.
U.S. Pat. No. 7,646,166 issued Jan. 12, 2010 to Koch et al., titled Method and Apparatus for Modeling Diffusion in an Electrochemical System, assigned to the assignee of this application and herein incorporated by reference, discloses a technique for modeling a vehicle battery circuit for determining battery SOC that includes a diffusion circuit element having a variable resistance to more accurately define the diffusion portion of the model circuit. The variable resistance is controlled in the diffusion circuit element based on the capacitance of a capacitor in an RC pair in that circuit element.